1. Field of the Invention
The present invention relates generally to a small twin spool gas turbine engine, and more specifically the cooling of the rear end bearings.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, the rotor shaft or shafts is supported for rotation by at least two bearings which include a forward or compressor end bearing and an aft or turbine engine bearing. Both radial and axial loads must be absorbed by the bearings. It is typical in the larger gas turbine engines of the prior art to use a ball bearing in the compressor end and a roller bearing in the turbine engine. The ball bearing can absorb both radial and axial loads and therefore acts as the thrust bearing for the rotor shaft. Because of the high temperatures that occur in the turbine end, the rear bearing must be capable of allowing axial displacement between the outer race support housing for the bearing and the bearing itself. This is why roller bearings are used in the turbine end. However, roller bearings only provide radial load absorption and no axial load. If a roller bearing is subject to an axial load, the rollers will start to rotate and then wobble during rotation of the bearing. This is a very undesirable situation and usually will result in the bearing blowing itself apart.
In a small gas turbine engine of bellow around 300 pounds thrust, the turbine end of the engine is exposed to very hot temperatures. The turbine end bearings are usually cooled by an external supplied cooling fluid such as a wet lubricant. Using a wet lubricant to provide cooling for the bearing requires a wet lubricant reservoir and the pumping and delivery system to circulate the liquid lubricant. This takes up additional space and adds weight to the engine. For a small gas turbine engine, this could add double the size and weight of the overall engine.
A typical gas turbine engine, especially of the small size as in the present invention, the rear end bearings would be cooled by sealing these bearings in a compartment that is filled with a wet or liquid lubricant. Or, some small engines pass fuel through the bearings to produce lubrication and cooling, and then discharge the unburned fuel out from the engine. In these engines, a lubricant is required to be stored in the engine until the engine is used. For a missile that uses this type of engine, long time storage is a problem because the fuel or lubricant cannot be held for long before it degrades.